This invention relates to a method for rapid kill gas injection to gas phase polymerization reactors during power failures.
The Stevens et al U.S. Pat. No. 4,326,048, the entire disclosure of which is expressly incorporated herein by reference, describes a method for terminating a gas phase olefin polymerization by injecting a carbon oxide. The injection of carbon oxide may take place downstream of the polymerization reactor, e.g., in the recycle gas line (Note column 4, lines 29-33 of the Stevens et al U.S. Pat. No. 4,326,048). The gas phase olefin polymerization may take place in stirred bed reactors or fluidized bed reactors. An example of such a fluidized bed reactor is described in the Miller U.S. Pat. No. 4,003,712, the entire disclosure of which is incorporated by reference into the above-mentioned Stevens et al patent and is also expressly incorporated by reference herein.
The Charsley U.S. Pat. No. 4,306,044, the entire disclosure of which is incorporated herein by reference, describes a means for introducing carbon dioxide into a gas-phase olefin polymerization system to at least reduce the rate of polymerization. For example, the carbon dioxide may be injected manually when the polymerization does not respond to other means of control (Note column 3, lines 53-59 of the Charsley U.S. Pat. No. 4,306,044). One other means of control is by rapid venting of the reactor (Note column 1, lines 15-20 of the Charsley U.S. Pat. No. 4,306,044). Accordingly, the Charsley U.S. Pat. No. 4,306,044 suggests the introduction of carbon dioxide into a gas-phase olefin polymerization reaction while venting is taking place.
The Charsley U.S. Pat. No. 4,306,044 also suggests that the polymerization system may be equipped with a means for sensing a potentially dangerous condition and a means operative with this sensing means for automatically introducing carbon dioxide into the polymerization system. For example, the sensing means may comprise a motion switch on a stirrer shaft which detects failure of rotation of the stirrer.
The importance of being able to rapidly reduce the rate of reaction is pointed out, e.g., at column 1, lines 15-30, of the Charsley U.S. Pat. No. 4,306,044. More particularly, a run-away reaction can result in fusing of the polymer into a large mass which can only be broken up with great difficulty.
The Karol et al U.S. Pat. No. 4,302,566, the entire disclosure of which is expressly incorporated herein by reference, describes a fluidized bed reactor similar to that described in the aforementioned Miller U.S. Pat. No. 4,003,712. This Karol et al patent suggests that it is essential to operate the fluid bed reactor at a temperature below the sintering temperature of the polymer particles (Note column 12, lines 39-53 of the Karol et al U.S. Pat. No. 4,302,566). In normal operation, the temperature of the fluidized bed is primarily controlled by passing recycle gas through a compressor and then through a heat exchanger, wherein the recycle gas is stripped of heat of reaction before it is returned to the fluidized bed (Note column 11, lines 35-53).
If the compressor in the fluidized bed arrangement fails, e.g., due to electrical or mechanical failure, the cooling means for controlling the temperature in the bed becomes inoperative. Since olefin reactants are still in contact with active catalyst, exothermic heat of reaction causes the temperature of the bed to climb toward sintering temperatures in a run-away fashion. This situation would warrant an emergency shut down of the reactor. As suggested by the aforementioned Charsley U.S. Pat. No. 4,306,044, one might attempt to vent olefins from the reactor as fast as possible in an attempt to control the run-away reaction. In this regard, it is noted that the fluidized bed system as described in the Karol et al U.S. Pat. No. 4,302,566 is expressly provided with a venting system for shut down. (Note column 13, lines 56-58 of this Karol et al patent). However, there are practical constraints to the rate at which olefins can be vented from the reactor. More particularly, olefins cannot simply be released to the atmosphere for environmental reasons. Consequently, vented olefins are burned by passing same through a flare. Accordingly, further constraints result from the fact that the rate at which olefins are vented from the reactor cannot exceed the capacity of the flare in terms of the maximum rate at which olefins can be burned. Building flares of greater capacity involves greater construction costs. Furthermore, the size of the fireball when a large flare is operating at full capacity may be prohibitive for environmental or safety reasons.
It will be appreciated that the volume of carbon oxide kill gas which is needed to terminate olefin polymerization is practically negligible in comparison with the total volume of gas in a fluid bed reactor. Furthermore, when flow of recycle gas through the reactor ceases due to compressor failure, the pressure gradient across the bed becomes essentially zero. Accordingly, if kill gas is merely injected at a point below the bed, there is essentially no pressure gradient across the bed to induce the flow of kill gas through the bed. A pressure gradient across the bed can be induced by venting gas from the top of the bed, which venting would be expected in an emergency shut down operation. However, as previously mentioned, there are practical constraints as to the rate at which olefin containing gas can be vented from the reactor. Therefore, even when kill gas injection is accompanied by venting of the reactor, the rate of penetration of kill gas to catalyst particles may be relatively slow due to practical constraints associated with the rate at which the reactor can be vented. Unless the reaction is killed rapidly, the sintering temperature of the polymer particles may be exceeded. Accordingly, there is a need in the art for faster methods of killing the olefin polymerization reaction.